Syndiotactic 1,2-polybutadiene (SPBD) is a thermoplastic polymer which can be utilized in a wide variety of applications. For example, the incorporation of SPBD into rubber compositions which are utilized in the supporting carcass or innerliner of tires greatly improves the green strength of those compositions. Electron beam precure (microwave precure) is a technique which has gained wide commercial acceptance as a means of improving the green strength of synthetic elastomers which are used in building tires. However, electron beam precure techniques are costly. The incorporation of SPBD into blends of such synthetic elastomers can often improve green strength to the degree that electron beam precure is not required. The incorporation of SPBD into halogenated butyl rubbers which are utilized as the innerliner compositions for tires also greatly improves the scorch safety of such compositions. U.S. Pat. No. 4,274,462 disclosed that pneumatic tires having improved resistance against heat build-up can be prepared by utilizing SPBD fibers in their tread base rubber. SPBD can also be molded into a wide variety of useful articles.
Techniques for preparing SPBD by solution polymerization and by suspension polymerization are known in the art.
A process is disclosed in U.S. Pat. No. 3,778,424 for the preparation of syndiotactic 1,2-polybutadiene which comprises polymerizing 1,3-butadiene in an organic solvent in the presence of a catalyst composition composed of:
(a) a cobalt compound,
(b) an organoaluminum compound of the formula AlR.sub.3, in which R is a hydrocarbon radical of 1-6 carbons, and
(c) carbon disulfide.
U.S. Pat. No. 3,901,868 reveals a process for producing a butadiene polymer consisting essentially of syndiotactic 1,2-polybutadiene by the successive steps of:
(a) preparing a catalyst component solution by dissolving, in an inert organic solvent containing 1,3-butadiene, a cobalt compound, soluble in the organic solvent, such as (i) cobalt-.beta.-diketone complex, (ii) cobalt-.beta.-keto acid ester complex, (iii) cobalt salt of organic carboxylic acid, and (iv) halogenated cobalt-ligand compound complex, and an organoaluminum compound,
(b) preparing a catalyst composition by mixing the catalyst component solution with an alcohol, ketone or aldehyde compound and carbon disulfide,
(c) providing a polymerization mixture containing desired amounts of 1,3-butadiene, the catalyst composition and an inert organic solvent, and
(d) polymerizing 1,3-butadiene at a temperature of -20.degree. C. to 90.degree. C.
U.S. Pat. No. 4,429,085 discloses a process for producing syndiotactic 1,2-polybutadiene by suspension polymerization in an aqueous medium. In this aqueous polymerization process polybutadiene which has an essentially syndiotactic 1,2-microstructure is made by the steps of:
(A) preparing a catalyst component solution by dissolving, in an inert organic solvent containing 1,3-butadiene (a) at least one cobalt compound selected from the group consisting of (i) .beta.-diketone complexes of cobalt, (ii) .beta.-keto acid ester complexes of cobalt, (iii) cobalt salts of organic carboxylic acids having 6 to 15 carbon atoms, and (iv) complexes of halogenated cobalt compounds of the formula CoX.sub.n, wherein X represents a halogen atom and n represents 2 or 3, with an organic compound selected from the group consisting of tertiary amine alcohols, tertiary phosphines, ketones, and N,N-dialkylamides, and (b) at least one organoaluminum compound of the formula AlR.sub.3, wherein R represents a hydrocarbon radical of 1 to 6 carbon atoms:
(B) preparing a reaction mixture by mixing said catalyst component solution with a 1,3-butadiene/water mixture containing desired amounts of said 1,3-butadiene:
(C) preparing a polymerization mixture by mixing carbon disulfide throughout said reaction mixture, and
(D) polymerizing said 1,3-butadiene in said polymerization mixture into polybutadiene while agitating said polymerization mixture.
U.S. Pat. No. 4,751,275 discloses a process for the preparation of SPBD by the solution polymerization of 1,3-butadiene in a hydrocarbon polymerization medium, such as benzene, toluene, cyclohexane, or n-hexane. The catalyst system used in this solution polymerization contains a chromium-III compound which is soluble in hydrocarbons, a trialkylaluminum compound, and a dialkylphosphite, such as di-neopentylphosphite or di-butylphosphite.
Heretofore, blends of SPBD with rubbery elastomers have been prepared utilizing standard mixing techniques. For instance, SPBD can be mixed throughout a rubbery elastomer utilizing a Banbury mixer or a mill mixer. However, these standard mixing procedures have certain drawbacks. These drawbacks include high processing costs, polymer degradation, inadequate mixing, and process limitations. The processing equipment required in order to mix SPBD throughout rubbery elastomers by mastication is also expensive and very costly to operate. Such standard mixing procedures result in polymer degradation due to the high shearing forces and high temperatures which may be required for mixing. For instance, it is generally desirable to mix the SPBD throughout the rubbery elastomer at a temperature which is above the melting point of the SPBD. Accordingly, SPBD powder, which is utilized in tire innerliner or carcass compounds, is mixed into the compound utilizing standard mixing procedures at a temperature which is at least as high as the melting point of the SPBD being used. Since high mixing temperatures result in degradation of the rubbery elastomer being utilized as the innerliner or carcass compound, the melting point of the SPBD utilized has typically been limited to no more than about 190.degree. C. In order to limit polymer degradation, the SPBD utilized in such applications typically has a melting point of no more than about 160.degree. C. Even though the green strength of tire carcass compounds containing SPBD increases with the melting temperature of the SPBD, the higher mixing temperature associated with the higher melting SPBD makes its utilization very difficult because of the degradation that occurs utilizing standard mixing techniques. Furthermore, good dispersions of SPBD throughout rubbers are difficult to attain utilizing conventional mixing techniques.